The methods of data transmission by inductive coupling were originally developed for RFID (Radio Frequency Identification) applications, in order to read contactless electronic tags (products identification) or contactless badges (people identification). Because of the technological advances of these last years, these methods are now easily implemented and are considered as forming an outright technique of data transmission, which is not only reserved for identification applications but can also be used to create a RF transmission channel between two devices to make various kinds of proximity transactions.
Thus, although originally provided for allowing data exchange between a contactless integrated circuit and an inductive coupling reader, it seemed advantageous to apply these methods to the data exchange between inductive coupling readers. More particularly, two communication “models” have been considered to establish a RF transmission channel between two readers.
According to the first model, each reader alternately goes into an active mode or a passive mode depending on whether it must send data to the other reader or receive data from the other reader. Thus, the reader which is to send data emits an alternating magnetic field oscillating at a carrier frequency Fc (13.56 MHz according to ISO/IEC 14443 and ISO/IEC 15693 standards) and modulates the amplitude of the magnetic field as if it addressed a contactless integrated circuit. The reader which is to receive the data does not emit magnetic field and receives an antenna signal, image of the magnetic field.
According to the second model, one of the readers goes into the active mode and the other reader goes into the passive mode during all the communication. The transmission of data from the reader in active mode to the reader in passive mode is as previously ensured by modulating the amplitude of the magnetic field emitted. The data transmission from the reader in passive mode to the reader in active mode is ensured by modulating the impedance of the antenna coil of the reader in passive mode (that is also called retromodulation). This impedance modulation produces, in the antenna circuit of the reader in active mode, a load modulation signal which is mixed with the excitation signal and is extracted by means of adapted filters.
The passive mode of model 2, unlike the alternating active/passive mode of model 1, is called “emulation” mode. Indeed, the reader in passive mode “behaves” like a contactless integrated circuit, to receive data as well as to send data.
These methods of data transmission between inductive coupling readers thus offer various prospects of application. The industrial community calls them “NFC” (Near Field Communication) and a forum is dedicated to them (http://www.nfc-forum.org/home). NFC is defined as an open platform about a technology defined by the documents ECMA 340 http://www.ecma-international.org/publications/standards/Standard.htm), ETSI TS 102 190 V1.1.1 and ISO/IEC 18092. These documents define modulation schemes, coding, data transfer speeds, and frame formats for the RF interface of NFC readers, as well as communication initialization protocols required for the control of data collisions during the initialization, in active mode as well as in passive mode. However, the designation “NFC” in the present application more generally refers to a reader able to dialogue with another reader.
In parallel to the development of the NFC technology, it has become common to integrate inductive coupling readers in portable electronic devices like mobile phones, PDAs (Personal Digital Assistants) and portable computers. In these applications, the implementation of the NFC technique is of major interest since the portable electronic device can then exchange data with another portable electronic device by inductive coupling. The NFC technique thus provides the portable electronic devices with advanced functionalities in addition to GSM (Global System for Mobile Communications), Bluetooth® and WiFi functionalities, with the possibility of providing interdependences between these functionalities, for example the use of a NFC communication for initializing a Bluetooth® communication between two mobiles.
The implementation of the emulation mode in a NFC reader built in a portable electronic device offers as additional advantage the fact that the portable device becomes usable like a contactless chip card, in access control applications or identification applications, for example controlling the access to public transport services like metro, buses etc. As users must pay for these applications, they must be secure. That implies that the operation of the NFC reader is subjected to the presence, in the reader, of a removable security module, provided to the subscriber by the service provider(s) and which must be inserted in the reader.
In addition to these security constraints which impose the provision of a removable security module, it is wished that the NFC reader in the emulation mode can be electrically powered by the magnetic field emitted by the reader in active mode. Indeed, the battery of the device in which the reader is built in should not be used during a communication, to preserve the charge of the battery as well as to allow the emulation mode to be usable when the battery is completely empty. Indeed, if a mobile phone or a PDA is used as a contactless chip card to allow users to access a transport service, the access to this service must not depend on the state of the battery of the mobile phone or of the PDA.
However, this purpose is more difficult to achieve with a NFC reader equipped with a removable security module than with a simple contactless chip card, for some reasons that will be understood by referring to FIG. 1.
FIG. 1 schematically shows the architecture of a standard electronic system 5 for mobile phones integrating a secure NFC reader. The telephony part of the system 5 comprises a baseband circuit 11 (standard GSM telephony circuit) equipped with a UHF antenna 11′, and a SIM card 20 (“Subscriber Identification Module”). The circuit 11 is arranged on a mother board 10 and linked to the SIM card 20 by a serial data bus DB1, via a contact connector 1120 fixed to the mother board 10. Peripheral elements like the keyboard, the display and the body of the mobile phone are not shown.
The NFC reader comprises a reader circuit 12 mounted on the mother board 10, and an antenna circuit 13 having inputs A, B coupled to the reader circuit 12. The antenna circuit 13 comprises tuning capacitors and an antenna coil 13′, and is tuned on a carrier frequency Fc, for example 13.56 MHz according to the ISO/IEC 14443 and ISO/IEC 15693 standards which are mentioned as examples only in the present application. The reader circuit 12, the baseband circuit 11 and the SIM card 20 are electrically powered by an internal continuous voltage Vcci supplied by a battery 15 of the mobile phone.
The active mode of the reader is implemented by the reader circuit 12 which is designed to apply a frequency excitation signal Fc to the antenna circuit 13, to modulate the amplitude of the excitation signal, to detect and demodulate a load modulation signal, in order to exchange data with a contactless integrated circuit or a NFC reader in passive mode. In addition, the material resources of the circuit 11 (microprocessor, program memory, etc) are used to manage the applications in active mode, thanks to “applets” (application programs) saved in this circuit. The reader circuit 12 is thus linked to the baseband circuit 11 by a data bus DB2, generally of the type I2C (“Inter Integrated Circuit”) or SPI (“Serial Peripheral Interface”).
The secure emulation mode of the NFC reader is implemented by adding to these elements a passive interface circuit 16 and a removable security module 30 of the type mentioned above. The passive interface circuit 16 is arranged on the mother board 10 and the module 30 is linked to the circuit 16 by means of a contact connector 1030 attached to the mother board.
The passive interface circuit 16 is coupled to the antenna circuit 13, in parallel with the reader circuit 12. In presence of an alternating magnetic field FLD emitted by another reader 100 (schematically shown in FIG. 1), an antenna signal SA oscillating at a carrier frequency Fc appears at the terminals of the antenna circuit 13, by inductive coupling between the antenna coil 13′ and an antenna coil 110 of the other reader. The circuit 16 then extracts from the antenna signal a continuous voltage Vcce, a data signal SDTr and a primary clock signal CK0 of frequency Fc.
The clock signal CK0 and the data signal SDTr are sent to the module 30 through amplifier circuits TRIG1, TRIG2 (for example Schmitt triggers) which shape these signals. Similarly, the module 30 supplies a load modulation signal SIm which is shaped by an amplifier circuit TRIG3 before going through the connector 1030 so as to be applied to the load modulation switch SWIm.
Lastly, a voltage Vcc and an electrical potential reference GND (ground) are supplied to the module 30 via the connector 1030. More particularly, the voltage Vcc is supplied by a selection circuit receiving the extracted voltage Vcce and the internal voltage Vcci. The selection circuit is for example a diode circuit D1, D2 supplying the voltage Vcci when the voltage Vcce is absent or inferior to Vcci (ignoring the drop of voltage in the diodes D1, D2), if not, supplying the voltage Vcce.
The module 30 comprises a support 31, contact pads 32 onto which lean contact blades 1031 of the connector 1030, and a secure integrated circuit 33 embedded in the support 31 and having metallized contacts electrically linked to the contact pads 32. The integrated circuit 33 comprises a circuit RFCT, a microprocessor MP, a memory array MA and an identification register IDREG. The register IDREG comprises at least an anticollision identifier ID intervening during the opening of a RF transmission channel. The circuit RFCT comprises circuits for decoding the incoming data signal SDTr and coding the outgoing load modulation signal SIm. The memory array MA comprises programs for opening a RF transmission channel in passive mode and programs for managing one or more applications. The microprocessor MP is thus able to open a RF transmission channel in passive mode by exchanging RF frames with the reader 100, and by ensuring in particular the management of the initialization and anticollision steps. Once the transmission channel is opened, the microprocessor is able to manage a secure application by using the RF channel as means of transmission of application data. Similarly, the reader 100 comprises a software layer ensuring the opening of the RF transmission channel and a software layer ensuring the management of the application. The microprocessor MP can comprise various peripheral circuits, like a cryptography circuit and a random number generator, to allow it to manage the security aspects of the application.
For the NFC reader to operate in a completely passive way in emulation mode (the reader circuit 11 being powered off), the voltage Vcce and the current supplied by the passive interface circuit 16 must be sufficient to electrically power the circuits TRIG1, TRIG2, TRIG3 and the module 30.
However, this NFC reader consumes a non-negligible current, generally about a few milliamperes. This current considerably reduces the maximum communication distance with the reader 100, in relation to the one offered by a contactless chip card (the electric power recovered by inductive coupling depending on the inductive coupling ratio which varies with the communication distance). Thus, when the maximum communication distance is imposed by the specifications of the application and is superior to the possibilities of the reader, the emulation mode can only be implemented by taking some current from the battery 15 of the device.
In an embodiment shown in FIG. 2, the clock CK0 and data SDTr links are replaced by a unique link called “interface S2C”, which carries a data and clock carrier PSK-modulated (Phase Shift Keying) composite signal S(SDTr, CK0). The composite signal is supplied by a XOR gate which combines the signals CK0, SDTr supplied by the circuits TRIG1, TRIG2. This unique link is for example described in the document entitled “S2C Interface for NFC, Adding a general purpose interface between NFC and Secure IC to Secure NFC, 21-01-2005, Survey V1.00” (http://www.semiconductors.philips.com/acrobat/other/identification/S2C_survey—10.pdf).
However, this embodiment does not solve the problem of electrical overconsumption, the voltage Vcce and the current supplied by the passive interface circuit 16 further having to electrically power the XOR gate.
According to the observations, it appears that the amplifier circuits TRIG1, TRIG2, TRIG3 consume a large part of the electric energy extracted from the antenna circuit by the passive interface circuit 16, for example 30 to 50% of this energy. Indeed, these circuits must supply an output voltage and current sufficient to compensate electrical loss appearing in signals CK0 and SDTr when they go through the connector 1030, because of the connector stray capacitances which appear in particular between the contact blades 1031 of the connector and the contact pads 32 of the module 30. In addition, the electrical consumption of the circuit TRIG1 is high because the circuit switches at a high frequency, that is 13.56 MHz.